Heretofore, various methods have been developed for improving the bond strength of copper foil bonded by an adhesive to an insulating synthetic polymer substrate so as to form a laminate for use in the production of printed circuit boards. Also, methods have been developed for reducing undesirable laminate staining, or discoloration, believed to be caused by chemical reactions between the copper and acid components of the resin used to form the substrate. For example, U.S. Pat. No. 3,857,681 discloses that the above-indicated problems are resolved by treating the copper foil so as to produce a matte surface formed of a plurality of copper electrodeposits having certain defined characteristics and subsequently coating the matte surface with a thin layer of a metal which is substantially chemically inert to the substrate, for example, zinc which, when heated, will form a brassy layer with the underlying copper.
As disclosed in U.S. Pat. No. 4,572,768, however, there is one other threat to the quality of a printed circuit that results from processes used in its manufacture. This is undercutting, which is the removal during etching of the material under some or all of the foil that is protected by the photoresist that is applied to define the printed circuit. Removal of copper under the photoresist weakens bonding of the copper to the board. In extreme cases of undercutting, portions of the printed circuit may even become detached from the board. It was therefore found necessary that the treated foil be able to withstand the etchant sufficiently to produce an acceptably low degree of undercutting.
This latter patent discloses that the above and other problems can be overcome by a treatment for copper foil that is to be used for lamination to a board comprises, as disclosed in the first above patent, electrodepositing a dendritic layer of copper on the side of the foil that is to be laminated to the board and securing the dendritic layer by electrodepositing a gliding layer of copper over it. A barrier layer of a material which is substantially inert to the substrate is then electrodeposited over the gliding layer. The barrier layer is formed by means such as electrodeposition from a solution containing ions of zinc, nickel and antimony. The resulting nickel-zinc-antimony barrier layer improves the resistance to undercutting.
With the advent of miniaturization, very densely packed printed circuit boards are needed. Miniaturization often requires that the copper foil conductors, or track lines, of today's printed circuit board be as narrow as 5 mils or less. Manufacturers of electrodeposited copper foil for the electronic industry must, therefore, produce foils having very special properties. For example, copper foil track lines have to be very firmly anchored to the substrate, which typically is made of a resin such as epoxy, polyimide, phenolic or another like polymer.
Typically, to aid in providing a strong bond, one or both sides of the foil which is to be bonded to the substrate is subjected to a bonding treatment of the above-described type to form a matte surface which improves the bonding, or initial peel strength, between the foil and the substrate to which it is laminated. As referred to above, a barrier layer, usually composed of zinc or brass, is applied by electrodeposition to the matte surface of the foil, i.e., the outer layer which results from the above bonding treatment. This barrier layer prevents direct contact between the metallic copper and the resin of the substrate and mitigates laminate staining. Since the temperature of lamination is quite high and the duration of the process is long, e.g., 350.degree. F. for about one hour in the case of production of epoxy laminates, zinc atoms of the barrier layer tend to readily migrate into the copper matrix of the foil, and there is also a migration of copper atoms from the foil into the barrier layer. Thus, there is formed on the foil a layer of "thermal brass" having a zinc concentration which varies across its thickness.
It is the practice in manufacturing printed circuit boards from copper-clad laminate to form the image of the desired printed circuit pattern on the copper surface of the laminate by a photographic technique which leaves the desired pattern formed of a photoresist material on the surface of the copper. The photoresist material protects the underlying copper from an etching solution, usually a concentrated solution of cupric chloride and hydrochloric acid, used to remove copper foil from the unwanted areas. Following the etching, the boards are usually rinsed in a 10% solution of hydrochloric acid and then rinsed in water.
It will be appreciated that the above acidic solutions will act laterally, as well as perpendicularly, in dissolving the copper cladding. Since the brass barrier layer is more readily dissolved by the mineral acids than the copper, those lateral portions of the copper foil track lines immediately adjacent the substrate tend to be dissolved, so that the track lines are "undercut," which reduces the surface area of the copper foil bonded to the substrate. Consequently, the initial peel strength, or bonding strength, of the copper foil track line to the substrate is reduced.
Our tests conducted using 1 millimeter (mm) wide track lines showed a reduction of, for example, about 20% in the initial peel strength of copper clad laminates. This means that approximately 10% of the peel strength was lost on each side of the track line. Since 1 mm approximates 40 mils, this means that the width of the bonded area of the copper-polymer interface was reduced by 4 mils on each side of the track line. Therefore, in today's densely packed printed circuit boards using, e.g., 5 mil track lines, the track lines would become unanchored, or simply lifted from the polymeric substrate.
In view of the foregoing, it has become apparent to us that in order to supply the highest quality copper foil, especially foil suitable for densely packed printed circuit boards, the foil must have an improved ability to resist undercutting by the etching solution, and particularly by the hydrochloric acid.
While the methods disclosed in U.S. Pat. Nos. 3,857,681 and 4,572,768 provided significant advances in improving the quality of copper foil for use in printed circuit boards, still higher quality copper foil is required to meet the present day requirements. Even though the nickel-zinc-antimony barrier layer disclosed in U.S. Pat. No. 4,572,768 resists undercutting more than other types of barrier layers, still greater resistance to undercutting is required in order to meet present day requirements for copper foil, while at the same time providing a foil having good bonding characteristics and resistance to laminate staining.